Computed radiography (CR) systems using stimulable phosphor sheets enjoy broad acceptance as clinical imaging tools. In a CR system, radiation is passed through a subject and impinges upon a stimulable phosphor sheet, commonly referred to as a CR plate, that stores a portion of the radiation energy as a latent image. After exposure to the radiation, the stimulable phosphor on the CR plate is subsequently scanned using an excitation light, such as a visible light or laser beam, in order to emit the stored image.
Early CR scanning systems employ a flying-spot scanning mechanism, in which a single laser beam is scanned across the phosphor plate in a raster pattern. The resulting excitation that provides the stored image is then directed to a sensor, providing a single point of image data at a time. More recent CR systems have improved upon this earlier technique by providing a full line of image data at a time, offering advantages of faster throughput and lower cost and complexity over flying-spot scanners. As just one example, U.S. Pat. No. 6,373,074 (Mueller et al.) discloses a CR system that scans a full line of image data points at a time.
FIG. 1 shows the basic components of an optical scanning system 10 such as that described in U.S. Pat. No. 6,373,074. A linear array of light sources 12, typically an array of laser diodes, directs a linear scanning beam 14 onto a stimulable phosphor sheet 16 that has been irradiated and stores a latent X-ray image. One or more cylindrical lenses 18 are used to direct the highly asymmetric linear output beam along a line 20 on the surface of phosphor sheet 16. In a sensing head 22, collection optics 24 direct the stimulated light from line 20 on phosphor sheet 16 through an optical filter 26 and to a linear photodetector array 28, typically a charge couple device (CCD) array. Phosphor sheet 16 is indexed in direction D by a transport mechanism 60, such as a continuous belt or other indexing apparatus, to provide a scanning motion. In this way, phosphor sheet 16 is scanned past sensing head 22 to detect each line of the image stored thereon. The sensed image data is then processed by an image processor 30 that assembles a two-dimensional output image from each successive sensed line. The output image can then be recorded onto a writable medium such as a photosensitive film, or can be displayed.
There have been a number of solutions proposed for improving the overall performance of CR plate scanner optics, including the following:                U.S. Patent Application Publication No. 2003/0010945 (Ishikawa) discloses improvements to light projection apparatus for projecting a line of stimulating light from an array of laser diodes;        U.S. Patent Application Publication No. 2002/0096653 (Karasawa) discloses the use of condenser lens chromatic characteristics for isolating stimulated light from stimulating light provided from the array of laser diodes;        U.S. Patent Application Publication No. 2002/0056817 (Furue) discloses a more compact reading apparatus for obtaining the stored image from an irradiated stimulable phosphor sheet;        U.S. Patent Application Publication No. 2002/0040972 (Arakawa) discloses an optical reading head that employs a grid pattern for sensing each line of the stored image;        U.S. Patent Application Publication No. 2002/0100887 (Hagiwara et al.) discloses an improved photodiode arrangement in a scanning head for a stimulable phosphor sheet; and        U.S. Patent Application Publication No. 2001/0028047 (Isoda) discloses a system using conventional optical techniques with improvements to line sensor componentry for obtaining a larger percentage of the stimulated light.        
While there have been numerous improvements to apparatus and methods for obtaining the stored image on a CR plate, there is still need for increased efficiency and overall image quality. One widely recognized problem with existing CR plate readers relates to the need for improved image quality at image sensing circuitry (generally represented as linear photodetector array 28 in FIG. 1). The apparatus disclosed in U.S. Patent Application Publication Nos. 2002/0096653, 2001/0028047, 2002/0040972, and in U.S. Pat. No. 6,373,074, and elsewhere, for example, employ Selfoc™ lenses and provide 1:1 imaging. While this solution allows compact packaging of the sensing components and their support optics, it imposes a constraint on numerical aperture (NA). The Selfoc™ gradient index lens is characterized as having a low NA. The maximum f/# value for this type of lens is typically about f/2, which provides an NA of 0.25. Because collection efficiency of this lens is proportional to the square of the NA value, a low NA can significantly degrade overall system brightness. Yet another disadvantage of existing systems relates to the relatively low fill factor of the Selfoc lens array. Gaps between adjacent Selfoc™ lens elements limit the fill factor and further constrain light collection.
As a result of the overall inefficiency of the collection optics, the signal-to-noise (SN) ratio of conventional sensing systems is disappointing. Collecting light over a broader area, such as is disclosed in U.S. Patent Application Publication No. 2001/0028047 noted above, tends to further degrade the SN relationship, even when using two-channel sensing optics. Low collection efficiency also constrains the reading speed of the CR plate reader. In addition, these systems use 1:1 imaging, which may require two optical stages if an optical system other than a Selfoc™ lens is used, with correction for imaging aberration for each stage.
Thus it can be seen that while prior art solutions provide a CR plate reader with some capability, the need for improved light collection efficiency must be met for further improvements in reader sensitivity and overall performance.